Managing gain tilt in an optically amplified transmission system

ABSTRACT

An optical transmission system may include a combination of higher gain repeaters and lower gain repeaters, arranged to manage gain tilt. The higher gain repeaters and lower gain repeaters may be arranged to allow net gain to vary (e.g., above and below zero) within a predetermined acceptable net gain excursion. In one embodiment, the higher gain repeaters have a nominal gain value higher than a nominal span loss value and the lower gain repeaters have a nominal gain value lower than the nominal span loss value.

TECHNICAL FIELD

The present invention generally relates to optical telecommunications.More specifically, the present invention relates to managing gain tiltin an optically amplified transmission system by using a combination ofoptical amplifiers having different gain values.

BACKGROUND

Optical transmission systems, such as long-haul undersea opticaltransmission systems, may be used to transmit optical signals over longdistances. These long-haul optical transmission systems, however, sufferfrom signal degradation caused by many factors, for example, losses dueto thermal noise and scattering caused by optical fiber imperfections aswell as losses resulting from splicing during assembly. These and otherfactors combine to attenuate the optical signal propagating through thetransmission system.

To address this problem, optical signals are optically amplified atpredetermined locations along the transmission system. Opticaltransmission systems may include repeaters connected to lengths of fiberoptic cable. The repeaters may include optical amplifiers for amplifyingoptical signals transmitted in each direction in the transmissionsystem. A repeater together with a length of fiber optic cable generallyforms a transmission span, and multiple transmission spans form anoptical transmission segment. A system may be designed such that theamplification provided by each repeater (i.e., the repeater gain)compensates for the signal loss in the preceding transmission span(i.e., span loss).

In existing optimal transmission segment designs, all of the spans aregenerally designed to have the same nominal values with respect to gain,gain shape and noise contribution. For example, all amplifiers may beidentical and all cable lengths have the same nominal loss. Therepeaters may be designed to yield a flat gain across a given wavelengthrange for each transmission span. In existing optimized transmissionsegments, therefore, the nominal gain of each repeater is ideally equalto the nominal loss of each cable length to provide a net gain of aboutzero.

To reduce cost in traditional undersea optical transmission systems,repeaters were custom designed to support the longest possible repeaterspacing consistent with performance and capacity requirements for theproposed transmission segment. One result of this approach has been theproliferation of repeater gain codes, with each new transmission segmentdesign resulting in a new gain code that is optimized for thatparticular segment design. More recently, transmission systems have beenconstructed to make efficient use of existing inventory to meet customercapacity, schedule and performance requirements. When using inventoryrepeaters, however, the resulting segment designs may be suboptimal,either in repeater count or in segment gain shape.

Imbalances between repeater gain and span loss have been a problem insystems built from repeaters in inventory with stretched repeaterspacing as well as in new systems despite best efforts to match repeatergain and span loss. During system assembly, for example, uncertainty insplicing losses and the need to accommodate cables with losses differentthan the nominal design loss can result in net gains significantlyoffset from the ideal zero net gain for a transmission system.Imbalances may also be caused by losses added during system repairs, forexample, by adding extra cable and splices.

The imbalance between repeater gain and span loss may detrimentallyaffect the optical signal quality. In particular, when the span lossesin the assembled transmission spans exceed the repeater gain, negativegain tilt may occur. As used herein, gain tilt is the difference (e.g.,in dB) between the highest channel power and the lowest channel powerfor a given wavelength range. Negative gain tilt may adversely affectthe optical signal to noise ratio (OSNR) of the communication system,may consume dynamic range in pre-emphasis, and may compromise theoperation of a line monitoring system (LMS).

One solution to this problem includes monitoring gain tilt and managinggain tilt by adding line build-out attenuators (LBOs) to the opticalpath in couplings or joints or by adding tilt filters in a gainequalization joint (GEJ) as needed to maintain system gain tilt withinacceptable limits. A LBO may be added when the measured gain tilt ispositive and a GEJ may be added when the measured gain tilt is negative.When a GEJ is added, the cable span between repeaters may need to beshortened to compensate for the span loss resulting from the insertionof the GEJ itself. Alternatively, when a GEJ is inserted into a nominalloss span, loss is added to the span loss, which introduces negativegain tilt. Thus, adding a GEJ to a transmission system adds loss, whichdegrades OSNR, and increases cost.

Accordingly, there is a need for a method of managing gain tilt in anoptically amplified transmission system using a limited number ofnon-optimum repeaters. There is also a need for simplified design andmanufacturing of an optical communication system using a limited numberof repeater codes.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference should be made to the following detailed description whichshould be read in conjunction with the following figures, wherein likenumerals represent like parts:

FIG. 1 is a schematic diagram of an optical transmission system in whichgain tilt is managed, consistent with one embodiment of the presentinvention.

FIG. 2 is a graph illustrating the optimal repeater spacing versustransmission segment length based on exemplary computer simulations forone embodiment of an optical transmission system using dispersion slopemanaged fiber (DSMF).

FIG. 3 is a graph illustrating net gain versus repeater position fordifferent arrangements of higher gain repeaters and lower gain repeatersin an optical transmission system using dispersion slope managed fiber(DSMF).

FIG. 4 is a graph illustrating the effects of different net gainexcursions on the performance of the exemplary embodiments of theoptical transmission system using dispersion slope managed fiber (DSMF).

FIG. 5 is a graph illustrating the optimal repeater spacing versussegment length based on exemplary computer simulations for oneembodiment of an optical transmission system using non-slope-managedfiber types.

FIG. 6 is a graph illustrating net gain versus repeater position fordifferent arrangements of higher gain repeaters and lower gain repeatersin an optical transmission system using non-slope-managed fiber types.

FIG. 7 is a graph illustrating the effects of different net gainexcursions on the performance of the exemplary embodiments of theoptical transmission system using non-slope-managed fiber types.

DETAILED DESCRIPTION

Referring to FIG. 1, an optical transmission system 100, consistent withone embodiment of the present invention, is described in greater detail.In general, the optical transmission system 100 may include acombination of optical amplifiers or repeaters having different gainvalues, which are arranged to manage gain tilt in the system. Theconcepts described herein may be used to manage gain tilt in a newtransmission system being assembled or in the repair of an existingtransmission system. The transmission system 100 and concepts describedherein may be implemented in optical communications systems known tothose skilled in the art, such as long-haul wavelength divisionmultiplexing (WDM) and dense wavelength division multiplexing (DWDM)systems. One embodiment of the optical transmission system 100 mayinclude a series of transmission spans 102-1 to 102-n, each including afiber optic cable span 104-1 to 104-n coupled to a repeater 106-1 to106-n. Multiple transmission spans 102-1 to 102-n may be coupledtogether to form a transmission segment 110. Each of the cable spans104-1 to 104-n may include one or more optical fibers for transmittingoptical signals and each of the repeaters 106-1 to 106-n may include oneor more optical amplifiers for amplifying the transmitted opticalsignals. In one embodiment where the optical transmission system 100provides bi-directional transmission, each of the cable spans 104-1 to104-n includes at least two optical fiber spans for transmitting opticalsignals in each direction and each of the repeaters 106-1 to 106-nincludes at least two optical amplifiers for amplifying the signalstransmitted in each direction respectively on the fiber spans. Theoptical transmission system 100 may also include transmitter/receiverterminals 120, 122 for transmitting and/or receiving the opticalsignals.

Although the exemplary embodiments are described in the context of abi-directional transmission system, those skilled in the art willrecognize that the concepts described herein may also be applied to aunidirectional transmission system including optical fiber or cablespans and optical amplifiers or repeaters for transmitting andamplifying optical signals in one direction. It is to be understood thata system and method consistent with the invention may be incorporatedinto a wide variety of network components and configurations. Theillustrated exemplary embodiments herein are provided only by way ofexplanation, not of limitation.

The transmission segment 110 may be designed with uniform repeaterspacing (i.e., uniform lengths of cable spans 104-1 to 104-n) and with amix of different types of repeaters 106-1 to 106-n having different gainvalues. Each of the transmission spans 102-1 to 102-n may have a nominalspan loss value (L) based on, for example, the type of optical fiberused in the cable, the length of the cable span, and splices. Each ofthe repeaters 106-1 to 106-n may have either a relatively higher nominalgain value (G_(H)) or a relatively lower nominal gain value (G_(L))lower than the higher nominal gain value. A repeater with the highernominal gain value (G_(H)) is referred to as a higher gain repeater anda repeater with the lower nominal gain value (G_(L)) is referred to as alower gain repeater. As used herein, “nominal” means a designated ortheoretical amount that may vary from the actual amount. Transmissionspans and repeaters may be designed with nominal loss and gain values,respectively, but the actual loss and gain values may vary from thenominal values, as will be understood by those skilled in the art.

The nominal span loss value (L) may be generally the same for each ofthe transmission spans 102-1 to 102-n in a given transmission segment110. In one embodiment, the higher nominal gain value (G_(H)) of thehigher gain repeaters may be higher than the nominal span loss value (L)and the lower nominal gain value (G_(L)) of the lower gain repeaters maybe lower than the nominal span loss value (L). In another embodiment,the higher nominal gain value (G_(H)) of the higher gain repeaters maybe higher than the nominal span loss value (L) and the lower nominalgain value (G_(H)) of the lower gain repeaters may be about the same asthe nominal span loss value (L).

The transmission segment 110 may include a mix of higher gain repeatersand lower gain repeaters selected and arranged such that the accumulatedloss of the transmission segment 110 over several transmission spans102-1 to 102-n approximately matches the accumulated nominal gains ofthe repeaters 106-1 to 106-n in those transmission spans 102-1 to 102-n.In a transmission segment 110, an optical path starts at an input of thesegment 110 and ends at the output of the segment 110. For an opticalsignal moving in the direction of arrow 130, for example, the opticalpath starts at an input at a first end 132 of the segment 110 and endsat an output at a second end 134 of the segment. For an optical signalmoving in the opposite direction of arrow 136, the optical path startsat an input at the second end 134 of the segment 110 and ends at anoutput at the first end 132 of the segment 110.

The net gain (G_(N)) of the optical path may be defined at each pointalong the length of the optical path or segment 110. At a given point inthe optical path, the net gain (G_(N)) is the sum of the cabled fiberlosses (L) and the nominal gains (G_(L), G_(H)) of the repeaters 106-1to 106-n between the input of the optical path and that given point.Using the values of the net gain (G_(N)) at the outputs of the repeaters106-1 to 106-n, a gain vector G_(N)=[G_(N1), G_(N2), G_(N3), . . . ,G_(Nn)] may be defined, where:

$G_{Ni} = {{\sum\limits_{k = 1}^{i}G_{k}} + {\sum\limits_{k = 1}^{i}L_{k}}}$and G_(k) is the nominal gain of repeater k and L_(k) is the loss of thecable span k. The amount by which the net gain G_(N) varies above and/orbelow zero at points along the optical path is referred to as gainexcursion.

The repeaters with the nominal gains G_(H) and G_(L) may be arranged inthe transmission segment 110 such that the gain vector G_(N) stayswithin a predetermined acceptable gain excursion (e.g., close to zero),with a bias toward positive values for the net gains G_(Ni) at theoutput of each of the repeaters 106-1 to 106-n. This design principlemay be illustrated using the transmission segment 110 shown in FIG. 1.According to one arrangement, the first three repeaters 106-1, 106-2,106-3 may have the higher gain value (G_(H)), the fourth repeater 106-4may have the lower gain value (G_(L)) and the fifth repeater 106-5 mayhave the higher gain value (G_(H)). If the repeaters are optimallyarranged in this exemplary design, the net gains (G_(N1), G_(N2),G_(N3)) at the output of the first three repeaters 106-1, 106-2, 106-3would be positive and the net gain (G_(N4)) at the output of the fourthrepeater 106-4 would be zero or negative. According to an alternativearrangement, the fourth repeater 106-4 may have the higher gain value(G_(H)) and the fifth repeater 106-5 may have the lower gain value(G_(L)). If the net gains (G_(N1), G_(N2), G_(N3) G_(N4), G_(N5)) at theoutputs of all five repeaters 106-1 to 106-5 are positive in thisalternative arrangement, one skilled in the art will recognize that alower gain repeater should have been used earlier in the segment 110 tomaintain the net gain closer to zero.

Three different exemplary segment designs are considered to furtherillustrate how the repeaters having the higher and lower gain values maybe arranged to manage gain tilt and to optimize the optical path. Inthese exemplary designs, the length of the transmission segment 110 isabout 3150 km, the nominal span length is about 105 km, the nominal spanloss is about 22 dB, the number of repeaters is thirty (30), and therepeater gain values (or codes) are about 18 dB (G_(L)) and 23 dB(G_(H)). Table 1 illustrates the net gain vectors of the three differentsegment designs identified as Cases 1-3.

TABLE 1 Case 1 Case 2 Case 3 Nominal Nominal Nominal Repeater RepeaterNet Repeater Net Repeater Net Position Gain Gain Gain Gain Gain Gain 123 1 23 1 23 1 2 23 2 23 2 23 2 3 23 3 18 −2 23 3 4 23 4 23 −1 18 −1 518 0 23 0 23 0 6 23 1 23 1 23 1 7 23 2 23 2 23 2 8 23 3 18 −2 23 3 9 234 23 −1 23 4 10 18 0 23 0 18 0 11 23 1 23 1 23 1 12 23 2 23 2 23 2 13 233 18 −2 23 3 14 23 4 23 −1 23 4 15 18 0 23 0 23 5 16 23 1 23 1 18 1 1723 2 23 2 23 2 18 23 3 18 −2 23 3 19 23 4 23 −1 23 4 20 18 0 23 0 23 521 23 1 23 1 23 6 22 23 2 23 2 18 2 23 23 3 18 −2 23 3 24 23 4 23 −1 234 25 18 0 23 0 23 5 26 23 1 23 1 23 6 27 23 2 23 2 23 7 28 23 3 18 −2 183 29 23 4 23 −1 23 4 30 18 0 23 0 23 5

In Case 1 and Case 2, every fifth repeater has a lower gain value (i.e.,18 dB). In Case 2, the first lower gain repeater is in location three(3) whereas in Case 1, the first lower gain repeater is in location five(5). Case 2 is better optimized because the maximum excursion of netgain from zero (i.e., gain excursion) is smaller and because the netgain vector is the same from both ends of the segment. In Case 1, thenet gain is always zero or positive in the direction from the repeaterat location one (1) to the repeater at location thirty (30), but in theopposite direction, the net gain is always zero or negative because thegain of the first repeater in location thirty (30) is 4 dB lower thanthe span loss.

Case 3 illustrates a less optimal design where there are five highergain repeaters followed by one low gain repeater. In the less optimaldesign of Case 3, the net gain is always positive and grows to a highvalue (i.e., 7 dB), as compared to the maximum net gain of 2 dB for Case2. Thus, the gain excursion in Case 3 is higher than the gain excursionin Case 2. The net gain of 2 dB in Case 2 results in 1 dB positive gaintilt in 28 nm. While some exemplary embodiments may presently bedescribed, a person of ordinary skill in the art will appreciate thatother embodiments may be possible having different lengths of cable,different numbers of repeaters and/or different placements of repeaters.A person of ordinary skill in the art will also appreciate that othernominal loss values and nominal gain values and combinations of repeatergains and optical fiber span losses may be possible. Although thesegments described above use two different repeater types with twonominal gain values, other repeater types with other gain values (e.g.,a medium gain repeater) may also be used in the segments.

Nearly optimal transmission segments of different lengths may bedesigned from a relatively small set of repeater types (e.g., highergain repeaters and lower gain repeaters). The use of higher gainrepeaters and lower gain repeaters allows a number of advantages duringthe life cycle of the transmission system. During system assembly, thegain tilt may be managed by changing a repeater type at a location to arepeater type that brings the net gain back toward zero. If negativegain tilt is accumulated, for example, a lower gain repeater may bereplaced with a higher gain repeater. With an inventory of higher gainrepeaters and lower gain repeaters, gain tilt may be managed withouthaving to add GEJs and without having to add additional loss to theoptical path, thereby preserving OSNR.

The use of higher gain repeaters and lower gain repeaters also providesadvantages from the standpoint of spare repeaters and system repair. Ifthe higher gain repeaters having gain higher than the nominal span lossare used as the spare repeaters, the higher gain may compensate for someor all of the loss added to the system by the repair operation (e.g.,extra cable and splices). This may reduce or eliminate the need foradditional repeaters in the repaired system to preserve acceptable gainshape and OSNR. Using higher gain and lower gain repeaters to managegain tilt may also minimize the number of repeater types needed ininventory, thereby simplifying repeater manufacturing and support.Further, the use of different repeater types to manage gain tiltfacilitates system design using repeaters from multiple suppliers.

According to one method of designing and constructing an opticaltransmission segment, consistent with the present invention, a repeaterspacing (or nominal span length) is determined for the segment toachieve the desired system transmission performance (e.g., the OSNR,gain shape, path average power and Q factor). The repeater spacing maybe determined by considering the repeater spacing in a system where eachof the repeaters has a nominal gain that matches the span loss. Asdescribed in greater detail below, computer simulations may be performedto determine the repeater spacing or nominal span length using matchedrepeaters (i.e., nominal gain=span loss). Those skilled in the art willrecognize that a number of different methods may be used to determinethe repeater spacing or nominal span length for an optical transmissionsegment. The nominal span loss may be determined based on the nominalspan length and other factors such as the type of fiber and the splicesin the span.

The repeaters may then be coupled to the cable spans having the chosenspan length and arranged such that a net gain in the segment variesabove and below zero net gain within a predetermined acceptable net gainexcursion, as described above. A repeater may be chosen from higher gainrepeaters and lower gain repeaters and assigned to each location in thetransmission segment such that the accumulated loss of the cable overseveral spans approximates the accumulated nominal gains of therepeaters in those spans. The higher gain repeaters and lower gainrepeaters may be designed and/or selected based on the nominal span losscorresponding to the chosen repeater spacing or span length. Forexample, the higher gain repeaters may have a nominal gain value higherthan the nominal span loss value and the lower gain repeaters may have anominal gain value lower than the nominal span loss value.

Alternatively, the repeater spacing may be determined based on thenominal gain values of the repeaters that are available (e.g., inventoryrepeaters) for use in the optical transmission segment beingconstructed. For example, the repeater spacing or nominal span lengthmay be chosen such that the nominal span loss is higher than the gain ofsome of the available repeaters and lower than the gain of otheravailable repeaters.

A wide range of transmission segment lengths may be constructed usingonly a few repeater codes (i.e., repeater nominal gain values). Oneembodiment of the method uses a combination of higher gain repeaters andlower gain repeaters, equal in number to the repeaters in a system thatuses only one repeater code optimized for the specific segment length,capacity and fiber type. The higher gain repeaters and lower gainrepeaters may be combined and arranged such that the accumulation of netgain (i.e., repeater gain minus span loss) is managed to allow anacceptable net gain excursion that is unlikely to have a significantimpact on system performance. As shown in the exemplary systems andsimulations discussed below, net gain excursions of less than about 10dB may not cause a significant penalty.

As described below in connection with exemplary designs for an opticaltransmission system, computer simulations may be used to determinerepeater spacing (or span length) and to evaluate different arrangementsof repeaters. The simulations may be performed using existing opticalcommunication system simulators known to those skilled in the art.

FIGS. 2-4 illustrate simulation results for one example of atransmission system using transmission spans including dispersion slopemanaged fiber (DSMF), specifically Allwave® single-mode (SM) fiber andTrueWave® inverse dispersion fiber (IDF) available from OFS. TheAllwave® SM fiber has an attenuation of about 0.184 dB/km, anattenuation slope of about −0.00017 dB/km/nm, a dispersion at 1550 nm ofabout 16.8 ps/nm/km, and a dispersion slop of about 0.057 ps/nm². TheTrueWave® IDF fiber has an attenuation value of about 0.234 dB/km, anattenuation slope of about −0.00049 dB/km/nm, a dispersion at 1550 nm of−44.0 ps/nm/km, and a dispersion slop of about −0.120 ps/nm².

The fiber mix in the DSMF transmission spans may be approximately 70%Allwave fiber and 30% IDF fiber with an average fiber attenuation(excluding splice loss) of about 0.199 dB/km and an average dispersionof about −1.5 ps/nm/km. A compensation span including only Allwave fibermay follow every ten transmission spans and may include a gain shapecorrection filter to correct for accumulated gain shape resulting fromuncorrected repeater gain shape. In this example, the span loss budgetmay include 0.6 dB for cover-to-cable splices and 0.3 dB for themid-span Allwave-IDF splice and 0.4 dB for unplanned splices, one every30 km, each splice having a loss of 0.1 dB. With the assumed fiberproperties in this example, the average span loss (averaged over 11spans in one period of a dispersion map) is about 25.1 dB for 120 kmtransmission spans and is reduced by 1 dB for every 5 km reduction inthe span length.

In this exemplary simulation, narrow-bandwidth repeaters, with no gainflattening filters, are assumed and the available repeater bandwidth maybe about 18 nm. A maximum repeater power of 15.5 dBm is assumed and therepeaters may have a noise figure of about 4.5 dB. During the exemplarysimulation, the system is loaded with 64 channels at 33.33 GHz channelspacing and a line rate of 11.5 Gbits/s is assumed for transmission. Theexemplary simulation uses OOK (On/Off Key) modulation, full AM (100%modulation depth) and no phase modulation. The exemplary simulation usesfifteen channels, in clusters of 5, at the two edges and the center ofthe transmission band, to determine the overall performance of theexemplary segment.

Based on this exemplary system and simulation, the optimal repeaterspacing is determined to be the spacing that provides the lowestrepeater count to support 64 channels over digital line segments (DLSs)of varying lengths. As shown in FIG. 2, the optimal repeater spacings inthis exemplary system are, respectively, 120 km, 100 km, and 90 km forDLS lengths of 3000 km, 6000 km and 9000 km. This determination of theoptimal repeater spacing assumes that, for a given DLS, the repeaterspacing and repeater gain is the same throughout the system. FIG. 2 alsoshows the repeater spacing as a function of DLS length to support 16,32, 64, 96 and 128 channels. These simulation results indicate that arepeater spacing of 90 km or longer may be used to support 64 channelsover a DLS that is no longer than 9000 km with the above exemplary fibercharacteristics. Maintaining the repeater spacing below 120 km may keepmulti-path impairments to a tolerable level.

According to this exemplary simulation, an arrangement of repeaters isdetermined and evaluated for a 6000 km DLS using 100 km spans. If a 6000km DLS using 100 km spans is constructed using previous techniquesmatching repeater gain to span loss, 60 matching repeaters may be used,each having a gain of 21.2 dB and corresponding to a 100 km fiber span.Consistent with the present invention, each of the matching repeaters insuch a 6000 km system may be replaced with either a lower gain repeaterhaving a nominal lower gain value (G_(L)) of about 19.2 dB or a highergain repeater having a nominal higher gain value (G_(H)) of about 25.1dB. In this example, the lower nominal gain of 19.2 dB generallycorresponds to the span loss in a 90 km span and the higher nominal gainof 25.1 dB generally corresponds to the span loss of a 120 km span.

FIG. 3 shows the net gain excursion for two exemplary 6000 km DLSconstructions consistent with the present invention. In a 10 dB net gainexcursion construction, the first five repeaters were replaced with thelower gain repeaters (i.e., 19.2 dB) resulting in a net gain after thefifth repeater close to about −10 dB and the next five repeaters arereplaced with higher gain repeaters (i.e., 25.1 dB) resulting in the netgain after the fifth repeater of about 10 dB. This may be continued forfifteen spans maintaining a net gain excursion of about 10 dB. In a 20dB net gain excursion construction, the lower gain repeaters and highergain repeaters are arranged to maintain a net gain excursion of about 20dB.

FIG. 4 shows the system performance Q as a function of the net gainexcursion, including the linear Q (same for all channels in the band)which is based on noise accumulation in the chain of repeaters and theminimum and average Q of the simulated channels. This plot indicatesthat the exemplary system is tolerant to relatively large accumulatednet gain with no significant gain penalty for net gain excursions of 10dB and only 0.2 dB penalty for a net gain excursion of 20 dB.

FIGS. 5-7 illustrate simulation results for another example of atransmission system using transmission spans including non-slope-managedfiber types, specifically TrueWave-XL® large mode fiber (LMF) and RX®non-dispersion shifted fiber (NDSF) fiber available from OFS. In thisexample, the transmission spans are purely LMF and are compensatedperiodically with NDSF fiber. The LMF fiber has an attenuation of about0.214 dB/km, an attenuation slope of about −0.00022 dB/km/nm, adispersion at 1550 nm of about −2.8 ps/nm/km, and a dispersion slop ofabout 0.111 ps/nm². The NDSF fiber has an attenuation value of about0.196 dB/km, an attenuation slope of about −0.00017 dB/km/nm, adispersion at 1550 nm of 16.8 ps/nm/km, and a dispersion slop of about0.057 ps/nm². The other assumptions for the simulation are the same asdescribed above for the DSMF system design.

FIG. 5 shows the optimal repeater spacing as a function of the DLSlength. The simulation results indicate that the optimal repeaterspacings are 120 km, 115 km, 100 km and 75 km for DLSs of lengths 1500km, 2000 km, 4000 km and 6000 km, respectively. The dark line shows thenonlinear limit, beyond which the system performance is severelypenalized by nonlinear impairments.

According to this exemplary simulation, an arrangement of repeaters isdetermined and evaluated for a 4000 km DLS. If a 4000 km DLS isconstructed using matching repeaters, a repeater spacing of 100 kmcorresponding to a 22.3 dB span is optimal to support 64 channels.Consistent with the present invention, a combination of higher gainrepeaters with a nominal gain value of 26.9 dB and lower gain repeaterswith a nominal gain value of 16.9 dB may be used in the 4000 km DLShaving the repeater spacing of 100 km. In this example, the highernominal gain of 26.9 dB corresponds to the nominal span loss of a 120 kmLMF-NDSF span and the lower nominal gain of 16.9 dB corresponds to thenominal span loss of a 75 km LMF-NDSF span.

FIG. 6 shows the net gain maps for three exemplary 4000 km DLSconstructions providing net gain excursions of 5 dB, 10 dB and 20 dB.FIG. 7 shows the simulated system performance as a function of the netgain excursion for this exemplary LMF-NDSF system. As with the exemplaryDSMF system discussed above, the simulation results indicate that aninsignificant Q penalty is incurred with the use of two repeaters havingtwo different gain values (or codes) to replace a single repeater havingone optimal gain value, especially if the maximum net gain excursion ismaintained below 10 dB.

While the exemplary embodiments described above illustrate substantiallyperiodic arrangements of optical fiber spans, higher gain repeaters, andlower gain repeaters, a person of ordinary skill in the art willappreciate that higher gain repeater placement and lower gain placementin a cable assembly may be determined by the required characteristics ofthe individual fiber optic cable spans. Those skilled in the art willrecognize that other fiber types with different properties may be used.Although the examples described above show an insignificant impact onsystem performance at specific gain excursions, those skilled in the artwill recognize that other gain excursions may also be possible and maydepend upon the system parameters and characteristics.

According to an alternative embodiment, gain tilt may be managed usingstandard repeaters for a transmission span, higher gain repeaters andadded loss such as line-build outs (LBOs). According to this embodiment,the standard (or nominal) repeaters may be designed for the system usingcurrent practices where the nominal repeater gain is matched to thenominal span loss. Higher gain repeaters may be designed using the samegeneral components but with a higher gain, for example, with a longererbium doped fiber (EDF) when EDFAs are used.

Gain tilt may be measured to determine the proper arrangement ofstandard repeaters, higher gain repeaters and LBOs. If excessivenegative gain tilt is accumulated during system assembly, a standard ornominal repeater may be replaced with a higher gain repeater. Ifexcessive positive gain tilt is accumulated during system assembly, lossmay be added to one or more spans, for example, by adding one or moreLBOs either in the cable/repeater coupling or in a cable-to-cable splicebox. By using the higher gain repeaters and LBOs, the gain tilt may bemanaged without having to shorten the length of the transmission spanand without having to use a GEJ. The higher gain repeaters may also beused as the spare repeaters, which may reduce the likelihood ofrequiring additional repeaters in the wet plant in the event of a deepwater repair.

In summary, embodiments of the present inventions provide systems andmethods for managing gain tilt in optical systems. Consistent with oneembodiment, an optical transmission system includes a plurality ofhigher gain optical amplifiers and a plurality of lower gain opticalamplifiers. The higher gain optical amplifiers each have a highernominal gain value and the lower optical amplifiers each have a lowernominal gain value lower than the higher nominal gain value. A pluralityof optical fiber spans are coupled to the optical amplifiers, and thehigher gain optical amplifiers and the lower gain optical amplifiers arearranged to allow a net gain at different points in the opticaltransmission system to vary within a predetermined acceptable net gainexcursion.

Consistent with a further embodiment, an optical transmission systemincludes a plurality of transmission spans coupled together. Each of thetransmission spans has a nominal span loss value and includes a fiberoptic cable span and an optical repeater coupled to the fiber opticcable span. The optical repeater in each of the transmission spans haseither a higher nominal gain value higher than the nominal span lossvalue or a lower nominal gain value lower than the span loss value toallow a net gain at different points in the optical transmission systemto vary within a predetermined acceptable net gain excursion.

Consistent with yet another embodiment, a method for managing gain tiltin an optical transmission segment includes determining a nominal spanloss value for transmission spans in the optical transmission system andproviding a plurality of higher gain repeaters each having a highernominal gain value higher than the nominal span loss value and aplurality of lower gain repeaters each having a lower nominal gain valuelower than the nominal span loss value. The higher gain repeaters andthe lower gain repeaters are arranged in a transmission segment suchthat a net gain at different points in the optical transmission segmentvaries within a predetermined acceptable net gain excursion.

Consistent with a further embodiment, a method of constructing anoptical transmission segment includes determining a repeater spacing inan optical transmission segment based on a nominal span loss and nominalrepeater gain substantially equal to the nominal span loss. The methodalso includes providing a plurality of fiber optic cable spanscorresponding to the repeater spacing, a plurality of repeaters eachhaving a higher nominal gain value, and a plurality of lower gainrepeaters each having a lower nominal gain value lower than the highernominal gain value. The repeaters are connected to the fiber optic cablespans to form transmission spans. The higher gain repeaters and thelower gain repeaters are arranged such that a net gain at differentpoints in the optical transmission segment varies within a predeterminedacceptable net gain excursion.

Consistent with yet another embodiment, a method of managing gain tiltin an optical transmission segment includes providing an opticaltransmission segment comprising a plurality of fiber optic cable spansand a plurality of repeaters coupled to the fiber optic cable spans.Gain tilt in the optical transmission segment is monitored. If negativegain tilt is accumulated at a repeater location in the transmissionsegment, the repeater at the repeater location is replaced with a highergain repeater having a higher nominal gain value than the nominal gainvalue of the repeater being replaced.

Accordingly, a relatively small set of repeater codes (i.e., repeaternominal gain values) may be used to support a wide range of capacity,segment length and fiber type requirements, with no additional repeatersand essentially no degradation in system performance. This simplifiesthe design and manufacturing processes for optical communicationsystems.

It should be emphasized that the above-described embodiments of thepresent invention are merely possible examples of implementations,merely set forth for a clear understanding of the principles of theinvention. Many variations and modifications may be made to theabove-described embodiment(s) of the invention without departingsubstantially from the spirit and principles of the invention. All suchmodifications and variations are intended to be included herein withinthe scope of this disclosure and the present invention and protected bythe following claims.

1. An optical transmission system comprising: a plurality of higher gainoptical amplifiers, each having a higher nominal gain value across aplurality of optical signal channels; a plurality of lower gain opticalamplifiers, each having a lower nominal gain value across said pluralityof optical signal channels lower than said higher nominal gain value; aplurality of optical fiber spans coupling said optical amplifiers,wherein said higher gain optical amplifiers and said lower gain opticalamplifiers are arranged to allow a net gain across said plurality ofoptical signal channels to vary within a predetermined acceptable netgain excursion at an output of each of said higher gain opticalamplifiers and said lower gain optical amplifiers; and wherein a netgain vector in one direction through said optical fiber spans and saidoptical amplifiers is the same as a net gain vector in the otherdirection through said optical fiber spans and said optical amplifiers,said net gain vector being represented by values of net gain at outputsof said optical amplifiers, and wherein said net gain at a respectiveoutput of one of said optical amplifiers is the sum of cabled fiberlosses at the respective output and nominal gains of said opticalamplifiers at the respective output.
 2. The optical transmission systemof claim 1 wherein a nominal span loss value of each of said opticalfiber spans is lower than said higher nominal gain value and higher thansaid lower nominal gain value.
 3. The optical transmission system ofclaim 2 wherein said nominal span loss value of each of said opticalfiber spans is essentially the same.
 4. The optical transmission systemof claim 1 wherein said net gain at one end of said optical transmissionsystem is approximately zero.
 5. The optical transmission system ofclaim 1 wherein said predetermined acceptable net gain excursion is lessthan about 10 dB.
 6. The optical transmission system of claim 1 whereinat least one of said optical fiber spans includes a plurality of fibersspliced together.
 7. The optical transmission system of claim 1 whereinsaid repeaters have essentially the same spacing.
 8. The opticaltransmission system of claim 1, further comprising an opticaltransmitter for transmitting an optical signal through said opticalfiber spans.
 9. The optical transmission system of claim 8, furthercomprising an optical receiver for receiving an optical signaltransmitted through said optical fiber spans.
 10. The system of claim 1wherein said optical amplifiers are erbium doped fiber amplifiers. 11.An optical transmission system comprising: a plurality of transmissionspans coupled together, each of said transmission spans having a nominalspan loss value across a plurality of optical signal channels andcomprising a fiber optic cable span and an optical repeater coupled tosaid fiber optic cable span, wherein said optical repeater in each ofsaid transmission spans has either a higher nominal gain value acrosssaid plurality of optical signal channels higher than said nominal spanloss value or a lower nominal gain value across said plurality ofoptical signal channels lower than said nominal span loss value to allowa net gain across said plurality of optical signal channels to varywithin a predetermined acceptable net gain excursion at an output ofsaid optical repeater in each of said transmission spans, wherein a netgain vector in one direction through said transmission spans is the sameas a net gain vector in the other direction through said repeaters andsaid optical transmission spans.
 12. The optical transmission system ofclaim 11 wherein said net gain varies above and below zero.
 13. Theoptical transmission system of claim 11 wherein said predeterminedacceptable net gain excursion is less than about 10 dB.
 14. The opticaltransmission system of claim 11 wherein each of said fiber optic cablespans includes at least first and second optical fiber spans fortransmitting optical signals in at least first and second directions.15. The optical transmission system of claim 14 wherein each of saidrepeaters includes at least first and second optical amplifiers foramplifying optical signals transmitted in said at least first and seconddirections.
 16. A method for managing gain tilt in an opticaltransmission segment, said method comprising: determining a nominal spanloss value for transmission spans in said optical transmission segment;providing a plurality of higher gain repeaters each having a highernominal gain value higher than said nominal span loss value; providing aplurality of lower gain repeaters each having a lower nominal gain valuelower than said nominal span loss value; and arranging said higher gainrepeaters and said lower gain repeaters in said optical transmissionsegment such that a net gain at different points in said opticaltransmission segment varies within a predetermined acceptable net gainexcursion and such that a net gain vector in one direction through saidrepeaters and said optical transmission spans is the same as a net gainvector in the other direction through said repeaters and said opticaltransmission spans.
 17. The method of claim 16 wherein said net gain atone end of said optical transmission segment is about zero.
 18. Themethod of claim 16 wherein said predetermined acceptable net gainexcursion is less than about 10 dB.
 19. A method of constructing anoptical transmission segment to manage gain tilt in an optical system,said method comprising: determining a repeater spacing in said opticaltransmission segment based on a nominal span loss and nominal repeatergain substantially equal to said nominal span loss; providing aplurality of fiber optic cable spans corresponding to said repeaterspacing; providing a plurality of higher gain repeaters each having ahigher nominal gain value; providing a plurality of lower gain repeaterseach having a lower nominal gain value lower than said higher nominalgain value; connecting said repeaters to said fiber optic cable spans toform transmission spans, wherein said higher gain repeaters and saidlower gain repeaters are arranged such that a net gain at differentpoints in said optical transmission segment varies within apredetermined acceptable net gain excursion, wherein connecting saidrepeaters comprises connecting a higher gain repeater at a locationwhere negative gain tilt accumulates; and connecting a line buildout(LBO) at a location where positive gain tilt accumulates.
 20. The methodof claim 19 wherein said lower nominal gain value is equivalent to anominal span loss value.
 21. The method of claim 19 wherein said highernominal gain value is higher than said nominal span loss value, andwherein said lower nominal gain value is lower than said nominal spanloss value.